Sheet transport device and image forming apparatus

ABSTRACT

A sheet transport device includes: a curved transport path for a sheet; and multiple endless belt pairs arranged along the transport path and having transport surfaces that are in contact with each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2022-052975 filed Mar. 29, 2022.

BACKGROUND (i) Technical Field

The present disclosure relates to a sheet transport device and an image forming apparatus.

(ii) Related Art

Japanese Unexamined Patent Application Publication No. 2013-49579 discloses a technique related to a sheet transport device.

Japanese Unexamined Patent Application Publication No. 2013-49579 discloses a sheet transport device including a first transport section that transports a sheet, and a second transport section that is disposed downstream of the first transport section in the sheet transport direction. The second transport section forms a nip part where the sheet transported by the first transport section is nipped and transported in a sheet transport direction that is different from the sheet transport direction in the first transport section. One of an opposing roller pair forming the nip part is a rotary-transport driving part that can transmit a driving force by rotating, and the other of the opposing roller pair is a belt transport part that is disposed on the outer side of the sheet transport path formed between the first transport part and the second transport part and that includes a belt driven by the rotary-transport driving part to transport the sheet to the nip part. The belt transport part includes the belt made of an elastic member, at least a pair of belt-holding rotary members that hold the belt in a rotatable manner, and a support member that supports the belt-holding rotary members in a rotatable manner. The support member supports the belt-holding rotary members such that the inter-axial distance of the belt-holding rotary members is maintained constant. The belt has a hardness of 40 to 80 degrees and has an extension rate of 10 to 5%, which is the amount of extension, in percentage, of the circumferential length of the belt stretched between the belt-holding rotary members.

SUMMARY

Aspects of non-limiting embodiments of the present disclosure relate to reducing damage to a sheet being transported, compared with a configuration in which there is no multiple endless belt pair in a curved sheet transport path.

Aspects of certain non-limiting embodiments of the present disclosure overcome the above disadvantages and/or other disadvantages not described above. However, aspects of the non-limiting embodiments are not required to overcome the disadvantages described above, and aspects of the non-limiting embodiments of the present disclosure may not overcome any of the disadvantages described above.

According to an aspect of the present disclosure, there is provided a sheet transport device including: a curved transport path for a sheet; and a plurality of endless belt pairs arranged along the transport path and having transport surfaces that are in contact with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiment of the present disclosure will be described in detail based on the following figures, wherein:

FIG. 1 shows the overall structure of an image forming apparatus that employs a sheet transport device according to an exemplary embodiment of the present disclosure;

FIG. 2 shows the structure of an image forming device of the image forming apparatus according to the exemplary embodiment of the present disclosure;

FIG. 3 shows the structure of the relevant part of the sheet transport device according to the exemplary embodiment of the present disclosure;

FIG. 4 shows a transport state of a recording sheet;

FIGS. 5A to 5C show the angle formed between a sheet-transport belt pair and a transport path;

FIGS. 6A to 6C show a mechanism for changing the belt length of a sheet-transport belt pair;

FIGS. 7A and 7B show a transport state of a recording sheet with the sheet transport device according to the exemplary embodiment of the present disclosure; and

FIGS. 8A to 8C show a transport state of a recording sheet with the sheet transport device according to the exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

An exemplary embodiment of the present disclosure will be described below with reference to the drawings.

Exemplary Embodiment

FIG. 1 shows the overall structure of an image forming apparatus that employs a sheet transport device according to an exemplary embodiment of the present disclosure.

Overall Configuration of Image Forming Apparatus

An image forming apparatus 1 according to the exemplary embodiment is, for example, a color printer. As shown in FIG. 1 , the image forming apparatus 1 includes: multiple image forming devices 10 that form toner images developed by toner, serving as developer; an intermediate transfer device 20 that holds the toner images formed by the image forming devices 10 and transports the toner images to a second transfer position, where the toner images are eventually second-transferred to a recording sheet 5, serving as a recording medium (sheet); a paper feeder 50 that stores and transports recording sheets 5 to be supplied to the second transfer position in the intermediate transfer device 20; and a fixing device 40 that fixes the toner image second-transferred to the recording sheet 5 by the intermediate transfer device 20. The image forming apparatus 1 includes a device body 1 a. The device body 1 a includes a support structural member and an exterior cover. One-dot chain lines in FIG. 1 show transport paths in the device body 1 a, along which a recording sheet 5 is transported. In the exemplary embodiment, the multiple image forming devices 10, the intermediate transfer device 20, and the fixing device 40 constitute an image forming section.

The image forming devices 10 include five image forming devices 10Y, 10M, 10C, 10K, and 10T, which form yellow (Y), magenta (M), cyan (C), black (K), and transparent (T) toner images, respectively. The five image forming devices 10 (Y, M, C, K, T) are arranged side-by-side in the horizontal direction inside the device body 1 a.

As shown in FIG. 2 , the image forming devices 10 (Y, M, C, K, T) each include a rotary photoconductor drum 11, serving as an image carrier. The photoconductor drum 11 is surrounded by: a charging device 12 that charges the circumferential surface (image carrying surface on which an image is to be formed) of the photoconductor drum 11 to a required electric potential; an exposure device 13 that radiates light onto the charged circumferential surface of the photoconductor drum 11 on the basis of image information (signal) to form an electrostatic latent image (for the corresponding color) having a potential difference; a developing device 14 (Y, M, C, K, T), serving as a developing unit, that develops the electrostatic latent image into a toner image with the toner in the developer of the corresponding color (Y, M, C, K, T); a first transfer device 15 (Y, M, C, K, T) that transfers the toner image to the intermediate transfer device 20; a drum cleaner 16 (Y, M, C, K, T) that removes a deposited substance, such as residual toner deposited on the image carrying surface of the photoconductor drum 11 after the first transfer, to clean the photoconductor drum 11.

The photoconductor drum 11 includes a grounded cylindrical or columnar base member and an image carrying surface formed on the circumferential surface thereof, the image carrying surface having a photoconductive layer (photosensitive layer) made of a photosensitive material. The photoconductor drum 11 is supported so as to be rotated in the arrow A direction by receiving a driving force from a driving unit (not shown).

The charging device 12 is a scorotron charger disposed at a distance from the photoconductor drum 11. A charging voltage is supplied to the charging device 12. The charging voltage is a voltage or a current with the same polarity as the charging polarity of the toner to be supplied from the developing device 14, when the developing device 14 performs reversal development. The charging device 12 may of course be a contact-type charging roller or the like, which is disposed in contact with the photoconductor drum 11.

The exposure device 13 performs deflection scanning of a laser beam corresponding to image information in the axial direction of the photoconductor drum 11. The exposure device 13 may be an LED print head that radiates light corresponding to the image information onto the photoconductor drum 11 with light-emitting diodes (LEDs), serving as multiple light-emitting elements, arranged along the axial direction of the photoconductor drum 11 to form an electrostatic latent image.

The developing devices 14 (Y, M, C, K, T) each include a housing 140 having a developer storage chamber and an opening at a position facing the photoconductor drum 11. The housing accommodates: a developing roller 141 that holds and transports the developer to a developing region facing the photoconductor drum 11; a stirring supply member 142, such as a screw auger or the like, that supplies, while stirring, the developer through the developing roller 141; a stirring transport member 143, such as a screw auger or the like, that transports, while stirring, the developer to the stirring supply member 142; a layer-thickness restricting member 144 that restricts the amount of developer (layer thickness) held on the developing roller 141; and the like. A power supply device (not shown) supplies a developing voltage between the photoconductor drum 11 and the developing roller 141 in the developing device 14. The five color developers are, for example, two-component developers each including a non-magnetic toner and a magnetic carrier.

The first transfer device 15 (Y, M, C, K, T) is a contact-type transfer device including a first transfer roller that is opposed to the circumference of the photoconductor drum 11 with an intermediate transfer belt 21 therebetween and rotates. The first transfer roller receives, from a power supply device (not shown), the supply of a first transfer voltage, which is a direct-current voltage with the opposite polarity to the charging polarity of the toner.

The drum cleaner 16 includes: a container-shaped body 160 having an opening; a cleaning plate 161 disposed in contact, at a certain pressure, with the circumferential surface of the photoconductor drum 11 after the first transfer to remove a deposited substance, such as residual toner; and a delivery member 162, such as a screw auger, that recovers the deposited substance, such as the toner, removed with the cleaning plate 161 and transports the deposited substance to a recovery system (not shown). The cleaning plate 161 is a plate-like member (for example, blade) made of, for example, rubber.

As shown in FIG. 1 , the intermediate transfer device 20 is disposed below the image forming devices 10 (Y, M, C, K, T) in the vertical direction. The intermediate transfer device 20 basically includes: the intermediate transfer belt 21 that revolves in the arrow B direction while passing through the first transfer positions located between the photoconductor drums 11 and the first transfer devices 15 (first transfer rollers); multiple belt-support rollers 22 to 24 that support the intermediate transfer belt 21 in a desired state from the inside thereof in a manner allowing rotation thereof; a second transfer device 30, serving as an example of a second transfer part, that is disposed on the outer circumferential surface (image carrying surface) of a portion of the intermediate transfer belt 21 supported by the belt-support roller 23 and that second-transfers a toner image on the intermediate transfer belt 21 to a recording sheet 5; and a belt cleaner 25 that removes the deposited substance, such as toner and paper dust, remaining on the outer circumferential surface of the intermediate transfer belt 21 after passing through the second transfer device 30.

The intermediate transfer belt 21 is an endless belt made of, for example, a synthetic resin, such as polyimide resin or polyamide resin, and a resistance adjuster, such as carbon black, dispersed therein. The belt-support roller 22 serves as an opposing roller for the belt cleaner 25, as well as a driving roller that is rotationally driven by a driving device (not shown). The belt-support roller 23 serves as an opposing roller for the second transfer device 30. The belt-support roller 24 serves as a surface forming roller that forms an image forming surface of the intermediate transfer belt 21.

As shown in FIG. 1 , the second transfer device 30 is a contact-type transfer device including a second transfer roller 31, to which a second transfer voltage is supplied. The second transfer roller 31 is in contact with and rotates on the circumferential surface of the intermediate transfer belt 21 at the second transfer position, which is an outer-circumferential-surface portion of the intermediate transfer belt 21 supported by the belt-support roller 23 in the intermediate transfer device 20. A power supply device (not shown) supplies, to the second transfer roller 31 or the belt-support roller 23 of the intermediate transfer device 20, a direct-current voltage, serving as a second transfer voltage, with the same polarity as or the opposite polarity to the charging polarity of the toner.

The fixing device 40 includes: a housing (not shown) having entry and exit ports for a recording sheet 5; a roller- or belt-type heating rotary member 41 that rotates in the counterclockwise direction and that is heated by a heating part such that the surface temperature thereof is maintained at a predetermined temperature; and a roller- or belt-type pressure rotary member 42 extending substantially in the axial direction of the heating rotary member 41 and in contact with the heating rotary member 41 at a predetermined pressure so as to be rotated in a driven manner. In the fixing device 40, the contact portion between the heating rotary member 41 and the pressure rotary member 42 serves as a fixing part, where necessary fixing processing (heating and pressing) is performed.

The paper feeder 50 is disposed below the image forming devices 10 (Y, M, C, K, T) and the intermediate transfer device 20. The paper feeder 50 includes one or more paper storage bodies 51 that accommodate a stack of recording sheets 5 of a desired size and type, and sending devices 52 that send out the recording sheets 5 one-by-one from the paper storage bodies 51. For example, the paper storage bodies 51 are attached to the device body 1 a such that a user can pull out the paper storage bodies 51 to the front side (i.e., the side surface to which the user faces when operating the apparatus) of the device body 1 a.

Examples of the recording sheets 5 include normal paper, thin paper, such as tracing paper, and overhead projector (OHP) sheets for electrophotographic copiers, printers, and the like. For even better smoothness of the image surface after fixing, it is desirable that the recording sheets 5 have as smooth surfaces as possible, and, for example, so-called thick paper having a relatively large grammage, such as coated paper formed by coating the surface of normal paper with resin or the like, art paper for printing, and the like, is also suitable.

As the needs are diversified these days, gold-color paper and silver-color paper having metallic (gold color, silver color, etc.) surfaces, and specialty recording media (hereinbelow, “specialty paper”), such as OHP sheets and coated paper having glossy surfaces, are used as the recording sheets 5. The specialty paper has a metallic surface or a glossy surface and is more likely to be degraded when the surface thereof gets a faint scratch, a cut mark, or the like before or after image forming, compared with normal paper.

A feeding transport path 57 including multiple paper-transport roller pairs 53 to 56, which transport a recording sheet 5 fed out of the paper feeder 50 to the second transfer position, and transport guides (not shown) is provided between the paper feeder 50 and the second transfer device 30. The paper-transport roller pair 56 disposed at a position immediately before the second transfer position in the feeding transport path 57 serves as, for example, rollers (registration rollers) for adjusting the transport timing of a recording sheet 5. A sheet transport belt 58 is provided between the second transfer device 30 and the fixing device 40 to transport, to the fixing device 40, the recording sheet 5 transported from the second transfer device 30 after the second transfer. An output transport path 62 including sheet output roller pairs 59 to 61, which discharge the recording sheet 5 transported from the fixing device 40 after fixing to a sheet output part (not shown) provided on a side surface of the device body 1 a, is provided near a sheet output port formed in the device body 1 a of the image forming apparatus 1.

The image forming apparatus 1 also includes a duplex unit 63 for forming images on both sides of a recording sheet 5. Instead of transporting the recording sheet 5 having an image formed on one side thereof to the sheet output part (not shown) with the sheet output roller pairs 59 to 61, the duplex unit 63 switches the sheet transport direction to the lower side with a transport-path switching part 62 a provided downstream of the sheet output roller pair 60 to guide the recording sheet 5 to a reverse transport path 66 including multiple reverse transport roller pairs 64 and 65. The forward rotation and reverse rotation of the reverse transport roller pair 65 can be switched. The reverse transport path 66 of the duplex unit 63 is connected to a duplex transport path 69 including multiple paper-transport roller pairs 68 via a duplex transport path 67. The recording sheet 5 transported to the reverse transport path 66 of the duplex unit 63 is transported again to the feeding transport path 57 via the duplex transport path 69.

FIG. 1 shows a controller 100 that generally controls the operation of the image forming apparatus 1. The controller 100 includes a central processing unit (CPU, not shown), a read-only memory (ROM), a random-access memory (RAM), a bus connecting the CPU, ROM, and the like, and a communication interface. A receiving unit 101 receives image information transmitted from an external host device, such as a personal computer, or an image reading device. A user interface 102 allows a user to input and set various conditions when the user operates the image forming apparatus 1. The user interface 102 includes a liquid crystal display panel or the like, via which the user sets various conditions and which displays messages and the like to the user.

Operation of Image Forming Apparatus

A basic image forming operation of the image forming apparatus 1 will be described.

A full-color-mode operation, in which the five image forming devices 10 (Y, M, C, K, T) form a full-color image composed of five color (Y, M, C, K, T) toner images, will be described below.

When the image forming apparatus 1 receives image information and instruction information of a full-color-image forming operation (printing) request from a host device, such as a personal computer, or an image reading device (not shown), the controller 100 actuates the five image forming devices 10 (Y, M, C, K, T), the intermediate transfer device 20, the second transfer device 30, the fixing device 40, and the like.

As shown in FIGS. 1 and 2 , in the image forming devices 10 (Y, M, C, K, T), first, the photoconductor drums 11 rotate in the arrow A direction, and the charging devices 12 charge the surfaces of the photoconductor drums 11 to a certain electric potential and a certain polarity (negative polarity in the exemplary embodiment). Next, the exposure devices 13 radiate light emitted on the basis of image signals corresponding to color components (Y, M, C, K, T) onto the charged surfaces of the photoconductor drums 11 to form, on the surfaces thereof, electrostatic latent images corresponding to the respective color components and having certain potential differences.

Next, the image forming devices 10 (Y, M, C, K, T) supply, with the developing rollers 141, toners of the respective colors (Y, M, C, K, T) charged with a certain polarity (negative polarity) to the corresponding electrostatic latent images formed on the photoconductor drums 11. The toners are electrostatically adhered to the electrostatic latent images to develop the electrostatic latent images. As a result, the electrostatic latent images formed on the photoconductor drums 11 are developed as five color toner images (Y, M, C, K, T), which have been developed with the toners of the corresponding colors.

Next, the color toner images formed on the photoconductor drums 11 of the image forming devices 10 (Y, M, C, K, T) are transported to the first transfer positions, where the first transfer devices 15 (Y, M, C, K, T) first-transfer the color toner images to the intermediate transfer belt 21, revolving in the arrow B direction, of the intermediate transfer device 20 such that the color toner images are sequentially superposed on one another.

In the image forming devices 10 (Y, M, C, K, T) after the first transfer, the drum cleaners 16 scrape off and remove the deposited substance to clean the surfaces of the photoconductor drums 11. By doing so, the image forming devices 10 (Y, M, C, K, T) can be used in the next image forming operation.

Next, in the intermediate transfer device 20, the revolving intermediate transfer belt 21 transports the first-transferred toner images held thereon to the second transfer position. Meanwhile, the paper feeder 50 feeds a recording sheet 5 into the feeding transport path 57 in accordance with the image forming operation. In the feeding transport path 57, the paper-transport roller pair 56, serving as registration rollers, feeds the recording sheet 5 to the second transfer position in accordance with the transfer timing.

At the second transfer position, the second transfer device 30 second-transfers the toner images on the intermediate transfer belt 21 together to the recording sheet 5. In the intermediate transfer device 20 after the second transfer, the belt cleaner 25 cleans the intermediate transfer belt 21 by removing the deposited substance, such as toner remaining on the surface of the intermediate transfer belt 21 after the second transfer.

The recording sheet 5 having the toner images second-transferred thereto is separated from the intermediate transfer belt 21 and is transported to the fixing device 40 via the sheet transport belt 58. In the fixing device 40, the recording sheet 5 after the second transfer is allowed to pass through the contact portion formed between the heating rotary member 41 and the pressure rotary member 42 rotating against each other, whereby necessary fixing processing (heating and pressing) is performed to fix the unfixed toner image to the recording sheet 5. Finally, the recording sheet 5 after fixing is discharged to the sheet output part (not shown) provided, for example, on the side surface of the device body 1 a by the sheet output roller pairs 59 to 61.

When images are to be formed on both sides of a recording sheet 5, instead of directly discharging the recording sheet 5 having an image on one side thereof to the sheet output part (not shown) via the output transport path 62, the sheet transport direction is switched to the lower side with the transport-path switching part 62 a to transport the recording sheet 5 to the reverse transport path 66 in the duplex unit 63. The reverse transport roller pair 65 switches the transport direction of the recording sheet 5 transported to the reverse transport path 66 in the duplex unit 63, and the recording sheet 5 is transported again to the feeding transport path 57 via the duplex transport path 69. After toner images have been transferred to the back surface of the recording sheet 5 at the second transfer position in the intermediate transfer device 20, the recording sheet 5 goes through the fixing processing in the fixing device 40 and is then discharged to the sheet output part (not shown) provided on the side surface of the device body 1 a by the sheet output roller pairs 59 to 61.

Through this operation, the recording sheet 5 having a full-color image composed of five color toner images is output.

Configuration of Sheet Transport Device

FIGS. 1 and 3 show the structure of the image forming apparatus that employs the paper transport device, serving as an example of a sheet transport device, according to the exemplary embodiment of the present disclosure.

As shown in FIG. 4 , the paper transport device 70 of the related art includes multiple paper-transport roller pairs 53 and 54 that transport a recording sheet 5 and guide members G that guide the recording sheet 5.

In the thus-configured related-art paper transport device 70, if the recording-sheet transport speed of the paper-transport roller pairs 53 and 54 varies or changes, at least one of the front and back surfaces of the recording sheet 5 may come into contact with or be rubbed by the guide members G or the like of the feeding transport path 57.

More specifically, as shown in FIG. 4 , when a recording sheet 5 with a curled leading end 5 a is transported, while the leading end 5 a is transported from the paper-transport roller pair 53 to the paper-transport roller pair 54, at least one of the front and back surfaces of the recording sheet 5 may come into contact with and be rubbed by the guide members G, or the like of the feeding transport path 57, or at least one of the front and back surfaces of the recording sheet 5 may come into contact with the guide member G of the feeding transport path 57 and then with the guide member G opposite thereto.

After the leading end 5 a of the recording sheet 5 has been transported from the paper-transport roller pair 53 to the paper-transport roller pair 54, as shown in FIGS. 5A to 5C, the back surface of the recording sheet 5 may come into contact with the guide member G of the feeding transport path 57, or the front surface of the recording sheet 5 may come into contact with the guide member G of the feeding transport path 57 due to, for example, a difference in the sheet transport speed between the paper-transport roller pairs 53 and 54. Moreover, after the leading end 5 a of the recording sheet 5 has passed through the paper-transport roller pair 53, the back surface of the recording sheet 5 may come into contact with the guide member G of the feeding transport path 57 again.

When the recording sheet 5 is normal paper, contact between at least one of the front and back surfaces of the recording sheet 5 and the guide members G of the feeding transport path 57 is not a serious issue. However, when the recording sheet 5 is specialty paper, such as metallic or glossy paper, contact with the guide members G is likely to cause a linear scratch on the front and back surfaces of the specialty paper, degrading the quality of the specialty paper, which is problematic.

To counter this problem, the paper transport device 70 according to the exemplary embodiment includes a curved transport path along which the sheet is transported and multiple endless belt pairs that are arranged along the transport path and in which the transport surfaces thereof are in contact with each other.

Specifically, as shown in FIG. 3 , the paper transport device 70 according to the exemplary embodiment includes, in the feeding transport path 57, multiple sheet-transport belt pairs 71 to 75, serving as an example of endless belt pairs, that are arranged along the curved transport path, along which the recording sheet 5 fed from the paper feeder 50 is transported to the second transfer position in the intermediate transfer device 20, and in which the transport surfaces thereof are in contact with each other.

The multiple sheet-transport belt pairs 71 to 75 include driving-side sheet transport belts 71 a to 75 a and driven-side sheet transport belts 71 b to 75 b. The driving-side sheet transport belts 71 a to 75 a of the multiple sheet-transport belt pairs 71 to 75 may be driven by their own driving sources or by one or more, but less than the number of the multiple sheet-transport belt pairs, driving sources by using driving gears, driving pulleys, etc. The multiple sheet-transport belt pairs 71 to 75 are arranged adjacent to one another at predetermined intervals (about 3 mm) along the curved transport path.

The sheet transport speeds of the multiple sheet-transport belt pairs 71 to 75 are set to be equal. More specifically, the multiple sheet-transport belt pairs 71 to 75 transport a recording sheet 5 along the curved transport path at a constant speed.

In the exemplary embodiment shown in FIG. 3 , the driving-side sheet transport belts 71 a to 75 a of the multiple sheet-transport belt pairs 71 to 75 are located on the outer circumferential side of the transport path, and the driven-side sheet transport belts 71 b to 75 b of the multiple sheet-transport belt pairs 71 to 75 are located on the inner circumferential side of the transport path.

The reason for the driving-side sheet transport belts 71 a to 75 a being arranged on the outer circumferential side of the transport path 57 is that, because the leading end of a recording sheet 5 transported from the upstream side comes into contact with the sheet transport belts 71 a to 75 a, which are located on the tangent side of the transport direction, that is, on the outer circumferential side of the transport path, the leading end of the recording sheet 5 is made to come into contact with the driving-side sheet transport belts 71 a to 75 a so as to be reliably transported to the next sheet-transport belt pairs 71 to 75.

As shown in FIGS. 5A to 5C, the multiple sheet-transport belt pair 71 a to 75 a are arranged such that the sheet transport directions thereof form a predetermined angle θ. Because the sheet-transport belt pairs themselves transport the recording sheet 5 linearly, the recording sheet 5 is curved or bent when passed between the adjoining sheet-transport belt pairs 71 to 75.

According to the study of the inventors with recording sheets 5 of various materials (grammages) and various angles between the adjoining sheet-transport belt pairs, as shown in FIGS. 5B and 5C, the recording sheets 5 with a grammage greater than 300 gsm are buckled or jammed, which is not preferable.

To counter this problem, in the paper transport device 70 according to the exemplary embodiment, the angle θ formed between the adjoining sheet-transport belt pairs 71 to 75 is set to less than or equal to 60°. Although setting the angle θ to less than 60° is desirable for ease of transporting sheets, the number of the sheet-transport belt pairs 71 to 75 increases, making the configuration complex, depending on the transport path. When the sheet transport direction is changed by 180°, at least four, and more desirably five or more, sheet-transport belt pairs are required.

In the paper transport device 70 according to the exemplary embodiment, by setting the angle θ formed between the adjoining sheet-transport belt pairs 71 to 75 to 60°, ease of transporting sheets and the simple structure are satisfied simultaneously.

In the paper transport device 70 according to the exemplary embodiment, the roller distances L1, L2, and L3, L4 of the sheet-transport belt pairs 71 to 75 are adjustable. As shown in FIGS. 6A to 6C, the pairs of rollers constituting the sheet-transport belt pairs 71 to 75 are rotatably attached to different support substrates 200 and 201. The two support substrates 200 and 201 are supported such that the pair of rollers move toward and away from each other through a stud 202, an elongated hole 203, and a fixing screw 204. The two support substrates 200 and 201 are rotatably supported through a rotation shaft 205 and an arc-shaped groove 206, such that the angle formed between the pair of rollers can be changed through a fixing screw 207. The belts are made of an elastic material.

Operation of Paper Transport Device

The paper transport device according to the exemplary embodiment suppresses damage to the sheet as below, compared with a case where a sheet transported along a curved transport path comes into contact with fixed guides.

Specifically, as shown in FIG. 1 , in the paper transport device 70 according to the exemplary embodiment, when an image forming operation is started by the controller 100, the paper feeder 50 starts to feed a recording sheet 5, and the recording sheet 5 is transported along the feeding transport path 57 to the second transfer position in the intermediate transfer device 20.

At this time, as shown in FIG. 3 , in the feeding transport path 57, the multiple sheet-transport belt pairs 71 to 75 are arranged so as to form a predetermined angle θ therebetween along the curved transport path.

As shown in FIG. 7A, a recording sheet 5 fed from the paper feeder 50 is passed from a paper-transport roller pair to the first sheet-transport belt pair 71, among the multiple sheet-transport belt pairs 71 to 75. At this time, the leading end of the recording sheet 5 transported by the paper-transport roller pair comes into contact with the driving-side sheet transport belt 71 a of the sheet-transport belt pair 71, which is located on the outer circumferential side. As a result, the leading end of the recording sheet 5 is reliably guided to the nip part between the driving-side sheet transport belt 71 a and the driven-side sheet transport belt 71 b by the driving-side sheet transport belt 71 a.

Then, the recording sheet 5 nipped between the driving-side sheet transport belt 71 a and the driven-side sheet transport belt 71 b, which constitute the first sheet-transport belt pair 71, is transported linearly in the transport direction of the first sheet-transport belt pair 71.

After that, the leading end of the recording sheet 5 is passed from the first sheet-transport belt pair 71 to the second sheet-transport belt pair 72. At this time, because the recording sheet 5 is passed from the sheet-transport belt pair 71 to the sheet-transport belt pair 72, the recording sheet 5 does not come into contact with (is not rubbed by) fixed guides member or the like. Thus, the front surface and/or the back surface of the recording sheet 5 are not damaged.

The recording sheet 5 is then passed from the second sheet-transport belt pair 72 to the third sheet-transport belt pair 73, as shown in FIGS. 8A to 8C.

Finally, the recording sheet 5 is passed from the fifth sheet-transport belt pair 75 to the paper-transport roller pair 55. Because the paper-transport roller pair 55 can be provided near the exit of the fifth sheet-transport belt pair 75, the risk of the recording sheet 5 being damaged due to contact (rubbing) with other fixed guide members and the like can be avoided.

Although the configuration in which the multiple paper-transport roller pairs 71 to 75 are arranged in a curved part of the feeding transport path 57 has been described in the exemplary embodiment, as shown in FIG. 1 , the configuration may of course be used in a curved transport path extending from the paper feeder 50 to the feeding transport path 57.

The foregoing description of the exemplary embodiments of the present disclosure has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical applications, thereby enabling others skilled in the art to understand the disclosure for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the following claims and their equivalents. 

What is claimed is:
 1. A sheet transport device comprising: a curved transport path for a sheet; and a plurality of endless belt pairs arranged along the transport path and having transport surfaces that are in contact with each other.
 2. The sheet transport device according to claim 1, wherein, in the plurality of endless belt pairs, an endless belt pair located on the downstream side has a smaller inclination angle with respect to the transport path than an endless belt pair located on the upstream side in the sheet transport direction.
 3. The sheet transport device according to claim 1, wherein the inclination angle of an endless belt pair located on the downstream side in the sheet transport direction with respect to the transport path is 60° or less.
 4. The sheet transport device according to claim 1, wherein the transport path includes a semicircular transport path for the sheet.
 5. The sheet transport device according to claim 4, wherein the plurality of endless belt pairs include at least four endless belt pairs arranged along the transport path.
 6. The sheet transport device according to claim 1, wherein the endless belt pairs each include a transport belt disposed on the outer circumferential side of the transport path and a transport belt disposed on the inner circumferential side of the transport path, the transport belt disposed on the outer circumferential side of the transport path having a greater circumferential length than the transport belt disposed on the inner circumferential side of the transport path.
 7. An image forming apparatus comprising: an image forming section that forms an image on a recording medium; and a transport part that transports the recording medium to the image forming section, the transport part including the sheet transport device according to claim
 1. 8. An image forming apparatus comprising: an image forming section that forms an image on a recording medium; and a transport part that transports the recording medium to the image forming section, the transport part including the transport device according to claim
 2. 9. An image forming apparatus comprising: an image forming section that forms an image on a recording medium; and a transport part that transports the recording medium to the image forming section, the transport part including the sheet transport device according to claim
 3. 10. An image forming apparatus comprising: an image forming section that forms an image on a recording medium; and a transport part that transports the recording medium to the image forming section, the transport part including the sheet transport device according to claim
 4. 11. An image forming apparatus comprising: an image forming section that forms an image on a recording medium; and a transport part that transports the recording medium to the image forming section, the transport part including the sheet transport device according to claim
 5. 12. An image forming apparatus comprising: an image forming section that forms an image on a recording medium; and a transport part that transports the recording medium to the image forming section, the transport part including the sheet transport device according to claim
 6. 